1. Field of the Invention
The present invention relates to a variable displacement pump which is preferably used in a hydraulic power steering apparatus for a motor vehicle or the like.
2. Description of the Related Art
As a variable displacement pump, there is a structure having a rotor fixed to a pump shaft inserted to a pump casing so as to be rotationally driven, and accommodating a plurality of vanes in a groove movable in a radial direction, an outer case fitted to a fitting hole within the pump casing, a cam ring supported to an oscillation supporting point provided in an inner peripheral surface of the outer case so as to freely oscillate and displace, forming a pump chamber with respect to an outer peripheral portion of the rotor, and forming first and second fluid pressure chambers with respect to the outer case, and a discharge flow rate control apparatus introducing a pressure in an upstream side of a main throttle provided in a pump discharge side passage to the first fluid pressure chamber, and introducing a pressure in a downstream side of the main throttle to the second fluid pressure chamber, wherein the discharge flow rate control apparatus has a switch valve apparatus which is actuated on the basis of a pressure difference between the upstream and downstream sides of the main throttle, and controls a supply fluid pressure to the first fluid pressure chamber in correspondence to a discharge flow rate of a pressure fluid from the pump chamber, as described in Japanese Patent Application Laid-open No. 2001-59481 (patent document 1).
In the variable displacement pump described in the patent document 1, the seal member with which the outer peripheral surface of the cam ring is brought into slidable contact is provided in the inner peripheral surface approximately in an opposite side to the portion in which the seal pin forming the oscillation supporting point of the outer case is provided, with respect to the pump shaft, and a space between the outer case and the cam ring is comparted into the first fluid pressure chamber and the second fluid pressure chamber by the seal pin and the seal member.
However, since the cam ring is structured such that an outer peripheral surface is formed as a complete round shape, an adhesion degree of the outer peripheral surface of the cam ring with respect to the seal member provided in the outer case is changed at a time when the cam ring is oscillated around the seal pin provided in the outer case. When the cam ring reaches an oscillation end, there is a case that the outer peripheral surface of the cam ring lowers the adhesion degree with respect to the seal member and lowers a sealing performance.